The present invention relates to a process for hydroconversion of hydrocarbon feedstocks which contain sulfur and nitrogen. More particularly it relates to a hydroconversion process utilizing multiple hydroconversion zones for reduced hydrogen, energy, and equipment costs.
The term "hydroconversion" is used here to connote a process in which hydrogen is reacted with a hydrocarbon on the surface of a heterogeneous hydroprocessing catalyst at process conditions. Example hydroconversion processes include hydrofining, hydrotreating and hydrocracking. The term "hydroconversion" is more particularly defined hereinbelow. The present invention is particularly directed to high pressure hydroconversion processes wherein the hydroconversion reaction zone is operated at a pressure above 500 psig.
In hydrofining, hydrotreating and hydrocracking reactions, an oil or other hydrocarbon feed is upgraded by chemical reactions carried out in the presence of hydrogen gas. Hydrofining is the mildest of these three types of hydroconversion processes. The term "hydrotreating" is generally applied to more severe hydroconversion processes than hydrofining, but often is used in a broad sense to include hydrofining. Typical hydrotreating reactions include desulfurization and denitrification of oil feeds. Heavy oil desulfurization is an important hydroconversion process and the process of the present invention is advantageously applied to such process. The term "hydrocracking" is generally used for more severe processes wherein more cracking of the oil feed occurs. However, there is not a sharp dividing line between these three types of hydroconversion processes. All three of these types of processes are well known and described in the literature, see, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Vol. 17, pages 201-206 and Vol 3, page 335.
The principal chemical reactions that occur in hydroconversion processes are cracking, hydrogenation, denitrification, desulfurization, demetalation and isomerization. These reactions are typically carried out by contacting a mixture of hydrogen and the feed hydrocarbons with a catalyst contained in one or more reactors at temperatures of 400.degree. F. to 850.degree. F. and pressures of 500 to 5,000 psig. The effluent from the hydroconversion reactor comprises unreacted hydrogen, converted and unconverted hydrocarbon materials (mainly hydrocarbons but often also small amounts of organic sulfur and/or nitrogen compounds), and product gases. The product gases include light hydrocarbons and contaminant gases, such as H.sub.2 S and NH.sub.3, generally produced by the hydrogenation of sulfur- and nitrogen-containing hydrocarbons.
In conventional hydroconversion processes, a combined feedstock comprising a hydrocarbon stream and hydrogen is caused to flow through a catalytic hydroconversion reaction zone in a downflow direction (see, for example, FIG. 10 of Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Vol. 17, p. 201). During such conventional processing, the reactions occurring near the top of the reaction zone are those reactions having a high reaction rate at the conditions in the reaction zone. When sulfur is present in the feed, hydrogen sulfide is generated relatively rapidly by the hydroconversion reactions. As the feedstock moves down through the zone, the hydrogen available for reaction becomes diluted with the hydrogen sulfide, ammonia and with the light gases generated by reaction. At the same time, the catalysts in the hydroconversion zone have reduced activity due to the presence of hydrogen sulfide and ammonia and are progressively contaminated through the hydroconversion zone. Consequently, using conventional processes, the more difficult reactions occur under conditions of lower catalyst activity and with lower available hydrogen purity.
The problems of the conventional processing are partially overcome by operating the reaction zone under conditions of countercurrent flow, as, for example, by introducing the liquid hydrocarbon feed to flow downward through the reaction zone, and introducing the hydrogen feed to flow upward through the same zone. P. Trambouze, "Countercurrent Two-Phase Flow Fixed Bed Catalytic Reactions," Chemical Engineering Science, Vol 45, No. 8, pp 2269-2275, 1990 describes such a countercurrent operation, lists commercial applications of the technology, and discusses the theoretical implications of this mode of operation.
U.S. Pat. No. 3,788,976, issued Jan. 29, 1974 to Kirk teaches a process for producing a refined mineral oil in a reaction vessel having two reaction zones and an intermediate zone, and with the hydrocarbon feed and hydrogen feed flowing in a countercurrent relationship with each other. The intermediate zone, intermediate between the two reaction zones, is disclosed as being useful for stripping the hydrogen sulfide formed in the first reaction zone from the hydrocarbon distillate. In the reaction zone below the intermediate zone, conversion reactions are maintained substantially free of sulfur and H.sub.2 S. As recognized by Kirk, one notable aspect of operating a hydroconversion zone with the hydrocarbon and hydrogen feeds in countercurrent flow is the stripping action of the hydrogen which removes hydrogen sulfide from the liquid phase in the reaction zone and/or intermediate zone. Thus, in the reaction zone near the hydrogen inlet under countercurrent flow conditions the liquid phase hydrocarbons, which are relatively free of sulfur, are hydroconverted over a catalyst relatively free of sulfur poisons in the presence of relatively pure hydrogen.
As hydrogen moves through a hydroconversion reaction zone in countercurrent flow to the hydrocarbon liquid phase, it also strips gaseous hydrocarbon products from the reacting liquid phase. In conventional hydroconversion processes, these gaseous products must be separated from the liquid products before they are further processed, at additional separation, compression, and reaction expense. In hydroconversion reaction systems with hydrocarbon and hydrogen flows in countercurrent relationship with each other, the processing equipment is much simplified and processing costs reduced.
U.S. Pat. No. 3,461,061, issued Aug. 12, 1969 to Stine, et.al. discloses a countercurrent reactor system, with a liquid phase heavy petroleum fraction passing downwardly through a reactor bed, and hydrogen rising upwardly in countercurrent contact with the petroleum fraction. The Stine process includes a second fixed bed catalytic reactor maintained under hydrogenating conditions through which a gaseous stream in vapor phase from the first reactor bed flows in a downward direction, cocurrent with added hydrogen.
However, in hydroconversion reaction processes with countercurrent flows of liquid and vapor phases, the vapor phase may sweep relatively unreacted feed components out the hydroconversion reaction zone before significant reaction occurs. Thus, GB 1,323,257, published Jul. 11, 1973 by Peck, et.al., discloses a hydrocarbon hydro-conversion process involving a reaction system with a heavy hydrocarbon charge stock flowing downward in a first reaction zone, and hydrogen flowing in the first catalyst zone countercurrent to the hydrocarbon stock, with conditions selected to maintain a lower-boiling hydrocarbon liquid derived from the charge stock in the second catalyst zone. U.S. Pat. No. 3,843,508, issued Oct. 22, 1974 to Wilson, et.al. discloses a similar process, with the additional feature that products from the reaction are additionally catalytically cracked.
However, selecting conditions to maintain a lower-boiling hydrocarbon liquid in the second catalyst zone puts severe limitations on the operation of that second zone. An improved process is much desired.